Flow effects in balanced steady state free precession imaging.

An analysis of the effect of flow on 2D fully balanced steady state free precession (SSFP) imaging is presented. Transient and steady-state SSFP signal intensities in the presence of steady and pulsatile flow were simulated using a matrix formalism based on the Bloch equations. Various through-plane flow waveforms and rates were modeled numerically considering factors such as the excitation slice profile and both in- and out-flow effects. Phantom measurements in an experimental setup that allowed the assessment of SSFP signal properties as a function of frequency offset and flow rate demonstrated that the computer simulations provided a suitable description of the effects of flow in SSFP imaging. A volunteer scan was performed to provide in vivo validations. For accurate modeling of SSFP signal intensities it is crucial to include effects such as imperfect slice profiles and, more importantly, "out-of-slice" contributions to the signal. Both simulations and experiments show that there can be considerably large-frequency offset dependent-signal contributions from flowing spins that have already left the imaging slice but still add to the SSFP signal. Although spins leaving the slice do not experience additional RF-excitation, gradient activity is not confined to the region of excitations and the balanced nature of the SSFP imaging gradients allows "out-of-slice" transverse magnetization to contribute to the total SSFP signal, effectively by broadening the slice thickness for flowing spins. This results in a frequency dependence of in-flow related signal enhancement and flow artifacts.